Apparatus for adding material to liquids

ABSTRACT

The turbidity of a liquid is measured before and after material has been added. The relative values of the measured turbidities are used to control the addition of the material so that the amount of material is added in any desired quantity or ratio relative to the liquid.

United States Patent Topol [451 Sept. 26, 1972 [54] APPARATUS FOR ADDINGMATERIA [56] References Cited TO LIQUIDS UNITED STATES PATENTS [72]Inventor: George J. Topol, 65 Charleston Ave H i Ontario Canada2,952,363 9/1960 Griswold ..210/193 4 1 Filed: y' 1970 3,06 ,816 11/1962Griswo d 210/193 X [21] App]. No; 33,179 Primary Examiner-John AdeeAttorney-Jeffers and Young Related US. Application Data [63]Continuation of Ser. No. 636,530, May 5, 1571 ABSTRACT 1967 abandoned IThe turbidity of a liquid is measured before and after 1 material hasbeen added. The relative values of the US. .a measured turbiditie areused to control the addition [51 Ilil. Cl. ..B0ld 37/02 of the materialso that h amount f i l i dd d Field Of arch ..2l0/96, 141, 143, 193, inany desired quantity or ratio relative to the liquid.

2 Claims, 11 Drawing Figures EEUQTJIEE? IO i '6 22 I2 I FlLTERED- H l3l4 l5 FlLTERED f TURBIDITY TURBIDITY |QU|D l METER METER FILTER v v 1NO. I N0. 2 W

CON TROL 24 SOLENOID UNlT VALVE 20 PATENTEDsms I972 3 6 93' 7 9 7 SHEET2 OF 5 EZR 7.5JTU

SIGNAL 0 SIGNAL 0 vALuEs VALUES 0 El SJJTU IOJTU TlME TURBIDITY CHANGETIME EXCESS OF SLURRY NORMAL SLURRY a DEFICIENCY 0F SLURRY 1 I qTuRBlDlTY INCREASE TURBIDITY TURBIDITY DECREASE (0) l0 (JTU) o E J JF C2R J .J

TI OPERATIONAL (d) E0 0 BAND +0 0F SOLENOID (e) SULENOID VALVE i T T2 T3OFF T4 T5 T T T T WE INVENTOR.

SLURRY PROPORTIONER WITH CHANGING TURBIDITY GEO TOPOL FI -EL BY ATORNEYS (0) FLOW RATE 50 (d) E 0 j OPEEQ'JSNAL uy. 0F SOLENOID 0NSOLENOID 5 ON OFF TIME SLURRY PROPORTIONER WITH CHANGING FLOW (u) A I 5-SLURRY A CEN- TION (b) El r F P F (d) A A A A A A A A A gggg \l X/ AL OFSOLENOID ON OFF TIME I INVENTOR SLURRY PROPORTIONER WITH CHANGING SLURRYCONCENTRATION GEORGE J TOPOL P'A'TENTEl'lsms m2 SHEET k 0F 5 CONTROLUNIT CONTROL UNIT SLURRY PUMP INVENTUR.

GEORGE J. TOPOL Ma/Z AT URI/Z PATENTED EPZ I 3.693.797

' SHEET 5 UF 5 FILTERED TURBIDITY LIQUID METER .FILTER INVENTOR.

GEORGE J. TOPOL RNEYS APPARATUS FOR ADDING MATERIAL T LIQUIDSThis-application is a continuation of application Ser. No. 636,530 filedMay 5, l 9.67 and now abandoned BACKGROUND OF THE INVENTION My inventionrelates to apparatus for adding a material to a liquid, and particularlyto such apparatus for automatically adding material to a liquid in anydesired quantity or ratio with respect to the quantity of liquid. Moreparticularly, my invention relates to apparatus for automatically addingfilter material in any desired quantity or ratio relative to thequantity of a liquid which is to be filtered.

In many filtering applications, the liquid to be filtered is suppliedwith a filter material such as diatomaceous earth or fullers earthbefore the liquid passes into the actual-filter structure. This addedfilter material assists in removing undesired foreign matter orcontaminants from the liquid as it passes through the filter structure.The filter material is usually added to the liquid asa slurry (sometimesreferred to as body feed) so as to provide better distribution of thefilter material throughout the liquid. The slurry is usually prepared ina tank which is arranged so that the slurry may flow intothe liquid tobe filtered, or so that the liquid to be filtered passes through thetankand picks up the slurry. The filter. material is one of the mostexpensive items in a filtering operation. Hence, it is important thatthe amount of slurry relative to the amount of liquid be controlled asclosely as possible to prevent an excess or unnecessary amount of filtermaterial to be added. Likewise, itis important that the amount of filtermaterial be sufficient to provide the desired degree of filtering. Aninsufficient amount of filter material provides an insufficientfiltering, with the result that the liquid must then be re-filtered, orthrown away.

Accordingly, an object of my invention is to provide an improvedapparatus for adding filter material to liquid that is to be filtered.

Another object of my invention is to provide apparatus for addingmaterial to a liquid in a predetermined or desired quantity or'ratiorelative to the quantity of liquid.

Iri prior filter applications, the slurry has generally been added infixed quantities. If necessary or desirable, these quantities werechanged by a manual adjustment. If the liquid to be filtered flows atvarying rates, or if the liquid to be filtered has varying amounts offoreign matter or contaminants, or if the slurry concentration changes,it is difficult, if not impossible, to manually adjust the amount offilter material added to provide the desired degree of filtering.

Accordingly, another object of my invention is to provide an improvedapparatus for adding a desiredquantity of filter material relative to aquantity of liquid and foreign matter or contaminants, despitevariations in the amount of liquid, or variations in the amount offoreign ,matter or contaminants in the liquid, or variations in theconcentration of filter material.

Another object of my invention is to provide an imof foreign matter orcontaminants in the liquid to be filtered.

Another object of my invention is to provide an improved apparatus forautomatically controlling the amount of filter material added to aliquid from a slurry tank so that the ratio of filter material to liquidand contaminants may be maintained as desired.

Another object of my invention is to provide improved apparatus forautomatically controlling the amount of filter material added from aslurry tank to a liquid in accordance with the amount of liquid to befiltered and/or in accordance with the amount of foreign matter orcontaminants in the liquid to be filtered.

Summary of the Invention Briefly, these and other objects are achievedin accordance with my invention by measuring the turbidity of the liquidat two locations. The first measurement is taken before the material hasbeen added to the liquid, and the second measurement is taken after thematerial has been added to the liquid. First and second signalsindicative of the two measurements are compared to produce a controlsignal indicative of the relative values of the two measurement signals.Since the second signal'indicates a higher turbidity because of theadded material, the second signal may be reduced to any desired ratio orproportion of its original value. The control signal is used toautomatically supply material to the liquid. In a preferred embodiment,when the control signal is positive, excess turbidity and material areindicated, so that the positive control signal is used to decrease theamount of material added, or is used to stop the addition of material.When the control signal is negative deficient turbidity and material areindicated, so that the negative control signal is used to increase theamount of material added, or is used to begin the addition of material.Thus, the control signal regulates the amount of material added in aclosed-loop type of servo system.

I BRIEF DESCRIPTION OF THE DRAWING The subject matter which I regard asmy invention is particularly pointed out and distinctly claimed in theclaims. The structure and operation of my invention, together withfurther objects and advantages, may be better understood from thefollowing description given in connection with the accompanying drawing,in which:

FIG. I shows a preferred embodiment of apparatus in accordance with myinvention for adding filter material to a liquid to be filtered;

FIG. 2 shows a schematic block diagram of the circuit elements which areutilized in the control unit shown in FIG. I;

FIGS. 3, 4, and 5 show diagrams illustrating how the measurement signalsand the control unit output signal vary with charges in the amount offilter material added;

FIGS. 6, 7, and 8 illustrate how the system of FIG. 1 operates withvariations in turbidity of the liquid, with variations in the flow ofliquid, and with variations in the concentration of filter material; and

FIGS. 9, 10, and 11 illustrate other systems of filters utilizing myinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a preferredembodiment of the apparatus for adding material to a liquid inaccordance with my invention. In FIG. 1, and in other subsequentfigures, I have illustrated my invention as being used to add filtermaterial to a liquid which is to be filtered. However, it is to beunderstood that my invention may be used in other applications where amaterial having some measurable turbidity is to be added to a liquidwhich also has some measurable turbidity. Likewise, my invention may beused to add material to a liquid where either the material or the liquidhas some measurable indication of concentration or color.

In FIG. 1, the liquid to be filtered is shown as being contained in atank 10. This liquid is drawn from the tank through a pipe ll and pumpedby a pump 12 through a line 13 to a turbidity meter No. l. The turbiditymeter No. I may be any known type of meter which senses the turbidity orcolor of a liquid, and produces an electrical output signal E inresponse to the turbidity or color sensed. Such meters are known in theart, and a typical turbidity meter utilizes a photosensitive device anda light source arranged so that the liquid to be measured passes betweenthe photosensitive device and light source. The liquid passes from theturbidity meter No. I through a line 14 to a turbidity meter No.2, whichmay be similar to meterNo. l. The turbidity meter No. 2 measures theturbidity of the liquid at this point, and produces an electrical outputsignal E, in response to the turbidity or color sensed. The liquid thenpasses from the turbidity meter No. 2 through a line 15 to a filter 16where it is filtered. The filter 16 may be any known type of filterwhich provides the desired type and degree of filtering for the liquid.The liquid leaves the filter 16 through a line 17 where it is utilized.The filtering of the liquid in the filter 16 is aided by the use of afilter material such as, but not limited to, diatomaceous earth orfullers earth. Since these filter materials are known, they will not bediscussed in this application. The filter material is placed in a slurrytank 18 where it is mixed with a suitable carrying liquid. In thisembodiment, liquid from the tank 10 is pumped by a pump 23 through aline 19, through a solenoid valve 20, and through a line 21 to theslurry tank 18. There the liquid is mixed with the filter material, andis supplied by a line 22 to the line 14 between the turbidity meter No.l and the turbidity, meter No. 2. The additional pressure from the pump23 forces the slurry through the line 22 and into the line 14 inaccordance with the condition of the solenoid valve 20. If the solenoidvalve is open, then slurry from the tank 18 flows into the line 14. Ifthe solenoid valve 20 is closed, then no slurry'rflows into the line.14. The output signals E and E from the turbidity meter No. l and theturbidity meter No. 2 are supplied to a control unit 24. This controlunit 24 compares the relative magnitudes or values of the signals E andE and produces an output signal E, which is indicative of, the relativevalues of these two measurement signals E and E The output signal E isapplied to the solenoidvalve 20 to open and close the solenoid valve 20in accordance with this output signal E As will be explained, the outputsignal E is, in the embodiment of FIG. 1, and on-off type of signal sothat the solenoid valve 20 is eitheropen or closed. However, it is to beunderstood that the output signal E may be a gradually changing signalwhich gradually changes the opening of a controlled valve so that lessor more liquid flows into the slurry tank 18, and less or more slurryflows into the line 14. As will be explained, whether slurry is pumpedinto the line 14 dependsupon the relative values of the measurementsprovided by the turbidity meter No. l and the turbidity meter No. 2.

FIG. 2 shows a schematic block diagram of the control unit 24 of FIG. 1.The two signals E and E: are, in this embodiment, direct currentsignals. If amplification of these signals E and E is needed, thesignals are applied to respective amplifiers which, if the signals aredirect current, should be linear. While the explanation of theinvention; is based on direct current signals, it is to be understoodthat alternating current signals can be used and compared in phase ormagnitude to produce a suitable output signal. The signal E, from theturbidity meter No. l is given a negative polarity that becomesincreasingly negative as the turbidity measured by the turbidity meterNo. 1 increases. The signal from the turbidity No. 2 is given a positivepolarity that becomes increasingly positive as the turbidity measured bythe turbidity meter No. 2 increases. It will be seen that the turbiditymeter No. 1 measures the turbidity of the liquid to be filtered withoutthe slurry, but that the turbidity meter No. 2 measures the turbidity ofthe liquid to be filtered with the turbidity of the slurry added intothe line 14. Hence, the signal E is applied to a ratio selector 28 whichreduces this signal E to a signal E having the desired ratio orproportion of its original value. This ratio or proportion determinesthe amount of slurry that will be added. For example, assume for aparticular liquid with a particular turbidity that it has beendetermined that filtermaterial representing 20 per cent of the totalturbidity in line 14 must be added. Then, the rate selector would be setso that its output signal E is 0.8 the signal E produced by theturbidity meter No. 2. The ratio selector 28 may be any type ofimpedance device (and may be calibrated) which reduces the signal E bysome desired proportion or to some desired ratio of its original valueor magnitude. The signals E, and E are applied to an integrator circuit29. This integrator circuit 29 may take any known form, and compares thetwo signals applied to its inputs and produces an output signal having amagnitude and polarity indicative of the relative magnitudes andpolarities of the applied input signals E and E This output signal E isapplied to the solenoid valve 20 in FIG. 1.

FIGS. 3, 4, and 5 show diagrams of the signal E, from the turbiditymeter No. l and the signal E from the ratio selector 28 and theturbidity meter No. 2 plotted against time under various conditions.FIGS. 3, 4, and 5 also show the output signal E, from the integrator 29under these conditions. In FIG. 3, it has been assumed has anincreasingly negative value which represents the difference between themagnitude of the signals E, and E This negative output signal E isapplied to the solenoid valve 20 to open'the valve 20 and start orincrease the amount of slurry feed. In FIG. 4, it has been assumed thatan excess of slurry is fed into the line 14. Under this assumedcondition, the magnitude of the signal E is considerably greater thanthe magnitude of the signal E. This condition is shown bythe outputsignal E having an increasingly positive polarity. This positive outputsignal I51 is applied to the solenoid valve 20 to close the solenoidvalve 20, or to decrease the amount of slurry feed.

In FIG. 5, two conditions have been assumed. At the beginning, or to theleft of FIG. 5, it has been assumed that the amount of slurry needed inthe line 22 is such that the turbidity of the slurry in the line 14 isthe same as theturbidity of the liquid to be filtered in the line 13. Inthis same assumption, a typical slurry turbidity might be JacksonTurbidity Units (JTU), and a typical liquid turbidity would then also be5 JTUs. The turbidity meter No. 1 would produce a signal E indicative of5 JTUs. However, since the liquid in the line 14 has the slurry added toit, the turbidity of the liquid with the slurry reaching turbidity meterNo. 2 would be JTUs. Thus, the output signal E from'the turbidity meterNo. 2 would indicate l0 JTUs. Since this is the desired condition, thesignal E from the turbidity meter No. 2 is reduced to 50 per cent of itsmagnitude by the ratio selector 28 so that the signal E provided by theratio selector 28 would have a magnitude indicative of 5 JTUs. Thus, asindicated at the left in FIG. 5, the output signal E would be zero, andno change in the solenoid valve would take place. At a later time, it isassumed that the turbidityof the liquid in the pipe 13 increases from 5JTUs to 10 JTUs. This increase is indicated by the change in the signalE to a negative value twice its previous value. At a slightly latertime,

because of the time required for the liquid to flow signal E increasesto a positive value indicative of 7.5

JTUs. At this point, the signal E is greater in mag- I nitude than thesignal E Thus, the output signal E becomes increasingly negative. Thisnegative signal indicates a deficiency of slurry, and would tend to openthe solenoid valve 20. When the valve 20 was properly set, both signalsE and E would be equal again.

FIG. 6 shows wave forms indicating how the magnitudes of the signals E Eand E change and how the solenoid 20 would operate for changes inturbidity over a period of time. The wave forms of FIG. 6 are plottedagainst a common time axis. FIG. 6a shows the liquid turbidity in JTUs,and it is assumed that the turbidity of the liquid is initially at 10JTUs. As shown in the output signal E becomes sufficiently negative tocause the solenoid control valve 20 to turn on or open. It has beenassumed that the solenoid valve 20 is an onoff type device having theoperational band indicated in FIG. 6d. When the solenoid valve 20 opens,it adds slurry to the line 14 with the result that the signal E becomesmuch more positive. This causes the output signal E to gradually becomemore positive. When the output signal E reaches the upper operationalvalue shown in FIG. 6d at the time T the solenoid valve 20 closes. Thus,the solenoid valve 20 has caused the slurry tank 18 to add slurrybetween the times T, and T FIG. 6e shows the operation of the solenoidvalve, with the dark sections indicating that the valve is on or open,and with the light sections indicating that the valve is off or closed.When the valve closes at the time T the signal E decreases in magnitudebecause of the reduced turbidity. The output signal E gradually becomesnegative until the operational band is reached. The solenoid opens againand adds more slur- This operation continues as shown until the time T.,when it is assumed that the turbidity of the liquid increases from 10JTUs to 20 JTUs as shown in FIG. 6a. The signal E becomes more negativeas shown in FIG.

6b. The solenoid valve is turned on, and remains turned on until thetime T5. The on or open time between the times T4 and T5 is greater thanthe previous on or open times (between the times T and T for example).This is because greater turbidity (produced by added slurry) is requiredbefore the signal E and the output signal E becomes sufficientlypositive to offset the now greater negative signal E so that the outputsignal E, can cause the solenoid valve to turn off or close. Once thesolenoid valve closes, it takes only a relatively short period from thetime T to the time T until the output signal E becomes negative andcauses the solenoid valve to turn on or open again. This operationcontinues until the time T, when it is assumed that the turbiditydecreases to 5 JTUs. This causes the signal E to become less negative,with the result that the signal E exceeds the signal E more easily. Theoutput signal E is now negative for a greater length of time. Thus, thesolenoid is on or open only for the relatively short period from thetime T to the time T and is off or closed from the relatively longperiod from the time T to the time T Thus, in considering FIG. 6, itwill be seen that for a turbidity of 10 JTUs, the solenoid operates tosupply slurry about one third of the time. For an increased turbidity,the solenoid operates to supply slurry for about two thirds of the time.For a decreased turbidity, the solenoid operates to supply slurry forabout one sixth of the time.

FIG. 7 shows how the system operates where the change or rate of flow ofthe liquid varies with time, but where its turbidity remainssubstantially constant. The rate of flow is shown in FIG. 7a and theturbidity indicated by the signal E from the turbidity meter No. l isshown in FIG. 7b. For a rate of flow of about 50 per cent, the solenoidvalve is on or open approximately 30 per cent of the time. When the rateof flow increases to per cent as shown in FIG. 7a, the signal E shown inFIG. 6c does not become as positive as before, since more liquid ispresent with the slurry. When the solenoid valve is turned on or opened,additional slurry is added. But with the increased flow, it takes alonger length of time for this additional slurry to bring the outputsignal E back to asufficiently positive value to close or turn'thesolenoid valve off. As shown in FIG. 7d, a longer length of time isrequired for the output signal E to reach the positive value to turn offor close the solenoid valve, so that the solenoid valve is on or openapproximately 60 per cent of the time. When the rate of flow isdecreased to the 25 per cent value shown, the decreased flow permits theslurry to raise the output signal E to a sufficiently positive valuerelatively quickly so as to close or turn the solenoid off. In this casethe solenoid valve is on or open approximately per cent of thetime..Thus, the apparatus taken into account the variations in the rateof flow of the liquid to be filtered. As shown in FIG. 7, the apparatusadjusts the slurry feed to compensate for changes in the rate of flow ofthe liquid to be filtered v The apparatus can also compensate forchanges in the slurry concentration, that is, the desired ratio offilter material to liquid will be maintained despite variations in theconcentration of filter material slurry. This is indicated in FIG. 8which shows the same type of wave forms as FIGS. 6 and 7 for a change inslurry con centration. As the slurry concentration goes up as shown inFIG. 8a, the signal E reaches a high positive value so that the outputsignal E reaches a positive value relatively quickly and turns thesolenoid off or closes it. Thus, the solenoid valve is closed arelatively high a positive value, so that the solenoid valve remainsopen for a relatively large part of the time..

In the preceding figures and discussion, it has been assumed thattheintegrator 29 is a'device having a reasonably long time constant ofoperation as indicated by the time to cover the operational band of thesolenoid, It is to be understood, however, that the integrator may beany type of integrating circuit having a time constant that varies froma relatively short time to a relatively long time. A relatively shorttime constant results in operation of the solenoid at a high rate andhence close or quick following of changes in rate of flow, turbidity, orslurry concentration. A relatively long time constant results in areduced rate of solenoid operation, but does not provide sufficientlyclose or quick following of the changes in turbidity, rate of flow, orslurry concentration. Hence, a time constant suitable for a particularapplication should be developed by trial and error until the bestoperation is achieved. If the outp'ut signal E is a gradually changinganalog type of signal, a valve can be provided which is opened avariable amount rather than either open or closed.

FIG. 1 has shown the operation of my invention with a slurry tank whichis supplied with liquid and pressure from the liquid to be filtered.Persons skilled in the art will appreciate that other systems may beprovided for the slurry tank. FIG. 9 shows one such system where theslurry tank receives the liquid=to be filtered and is supplied with dryslurry from a hopper 40. The slurry passes into a chute 41 where it ispermitted to flow into the slurry tank in accordance with the outputsignal E from the control unit. The slurry tank may be provided with asuitable mixer or agitator 42 as shown. FIG. 10

shows another system where the slurry tank is provided with the filtermaterial and it is pumped to the line by a slurry pump 45 which operatesin response to the output signal E And, FIG. 11 shows a system where theslurry tank is connected in a circulating line 50 by first and secondsolenoid valves 51, 52. The circulating line 50 is provided with acirculating pump 53 which circulates the liquid to be filtered from thefilter output back to a point between the two turbidity meters. Slurrycan be introduced into the liquid by opening the solenoid valve 51 andclosing the solenoid valve 52. The flow of slurry added can be stoppedby closing the solenoid valve 51 and opening the solenoid valve 52. Asshown, the control signal E -is applied to both solenoid valves 51, 52to provide this dual operation. The arrangement of FIG. 11 isparticularly desirable in that the total flow to the filter remainsessentially constant and does not fluctuate.

-It will thus be seen that my invention provides improved apparatus forcontrolling the amount of material added to a liquid. My apparatus canbe used to add almost any type of material to a liquid as long as eitherthe material or the liquid has a measurable degree of turbidity orconcentration. My apparatus provides automatic and accurate addition ofthe material, thus saving on the amount of material added. The apparatusutilizes a feedback control, and thus provides the desired ratio oramount of material despite changes in the turbidity of the liquid to befiltered, despite variations in the rate of flow of the liquid, anddespite variations in the concentration of the material to be added. Theapparatus thus reduces the necessary maintenance and attention for theslurry tank, and also permits a relatively small slurry tank to be used.Such a small slurry tank can be activated or operated by any suitableapparatus, such as the pressure of the liquid to be filtered, or by apump or some other source of pressure; And finally, the apparatus isautomatic and does not require attention. Persons skilled in the artwill appreciate that modifications may be made to my invention. Forexample, the apparatus can be used in other applications besides thefiltering embodiments shown.

For example, such apparatus might be in a chemical plant where an acidis to be diluted, or where a dye must have a certain concentration.Therefore, while my invention has been described with reference toparticular embodiments, it is to be understood that modifications may bemade without departing from the spirit of my invention or from the scopeof the claims.

I claim:

1 Apparatus for adding filter material and the like to a liquid flowingin a system, comprising:

a. first means positioned at a first location in said system for sensingthe turbidity of said liquid at said first location and producing afirst electrical signal having a voltage magnitude that varies as afunction of the turbidity sensed;

b. second means positioned at a second location in said systemdownstream from said first location for sensing the turbidity of saidliquid at said second location and producing a second electrical signalhaving a voltage magnitude that varies as a function of the turbiditysensed;

c. a source of filter material having an outlet coupled into said systembetween said first location and said second location, and having aliquid inlet coupled to said system upstream from said filter materialoutlet, said source including solenoid means for controlling the amountof filter material suppliedto said outlet and to said system;

d. utilization means for said liquid coupled to said system downstreamfrom said second location;

e. an electrical comparison circuit having first and second inputcircuits and an output circuit for comparing said first and secondsignals applied to said first and second input circuits and producing anelectrical output signal at said output circuit that varies in voltageand polarity as a function of the relative characteristics of said firstand second signals applied to said first and second input circuits; I

f. means respectively coupling said first and second input circuits tosaid first and second sensing means including ratio means in circuitwith said second input circuit and said second sensing means to supply aselectable fraction of said second electrical signal to said secondinput circuit;

g. and means coupling said output circuit to said control means of saidsource.

2. The apparatus of claim 1 wherein said solenoid means is operative inresponse to a control signal of one polarity and inoperative in responseto a control signal of an opposite polarity and wherein the electricalcomparison circuit comprises an integrator adapted to generate as itssaid electrical output signal the integral of the difference between thesaid first electrical signal and the said selectable fraction of thesecond electrical signal.

1. Apparatus for adding filter material and the like to a liquid flowingin a system, comprising: a. first means positioned at a first locationin said system for sensing the turbidity of said liquid at said firstlocation and producing a first electrical signal having a voltagemagnitude that varies as a function of the turbidity sensed; b. secondmeans positioned at a second location in said system downstream fromsaid first location for sensing the turbidity of said liquid at saidsecond location and producing a second electrical signal having avoltage magnitude that varies as a function of the turbidity sensed; c.a source of filter material having an outlet coupled into said systembetween said first location and said second location, and having aliquid inlet coupled to said system upstream from said filter materialoutlet, said source including solenoid means for controlling the amountof filter material supplied to said outlet and to said system; d.utilization means for said liquid coupled to said system downstream fromsaid second location; e. an electrical comparison circuit having firstand second input circuits and an output circuit for comparing said firstand second signals applied to said first and second input circuits andproducing an electrical output signal at said output circuit that variesin voltage and polarity as a function of the relative characteristics ofsaid first and second signals applied to said first and second inputcircuits; f. means respectively coupling said first and second inputcircuits to said first and second sensing means including ratio means incircuit with said second input circuit and said second sensing means tosupply a selectable fraction of said second electrical signal to saidsecond input circuit; g. and means coupling said output circuit to saidcontrol means of said source.
 2. The apparatus of claim 1 wherein saidsolenoid means is operative in response to a control signal of onepolarity and inoperative in response to a control signal of an oppositepolarity and wherein the electrical comparison circuit comprises anintegrator adapted to generate as its said electrical output signal theintegral of the difference between the said first electrical signal andthe said selectable fraction of the second electrical signal.